The document discusses evaluating and managing penicillin allergy. It finds that over 90% of patients labeled penicillin-allergic can tolerate penicillin after skin testing by an allergist. Skin testing is the best way to diagnose IgE-mediated allergy and has a high negative predictive value. If skin testing is negative, a graded challenge is recommended. If positive, an alternative antibiotic should be used or desensitization considered. Cross-reactivity between penicillin and cephalosporins is low at around 3%. Proper evaluation can help identify patients able to safely receive penicillin-class antibiotics.

1. Anwer G Almosewi

2. Penicillin allergy: A practical guide
for clinicians

3. After undergoing a complete evaluation by a board-
certified allergist , including skin testing, 90% of
patients labeled as “penicillin-allergic” are able to
tolerate penicillin.

4. The prevalence of reported penicillin allergy is 10% in
the general population. However, more than 90% of
these patients are found not to be allergic to penicillin
after skin testing.

5. In patients found to have penicillin allergy, the
frequency of positive results on skin testing decreases
by 10% per year of avoidance. Therefore, 80% to 100%
of patients are expected to test negative for penicillin
allergy by 10 years after their reaction.

6. Skin testing for penicillin allergy is only useful for type
1 IgE-mediated reactions

7. The rate of cross-reactivity between penicillin and
cephalosporins is approximately 3%.

8. MOST PATIENTS who report that they are allergic to
penicillin can ultimately receive penicillin or a
penicillin-type antibiotic again after an appropriate
evaluation and, possibly, treatment.

9. This course of action decreases the need for broad-
spectrum antibiotics,1–4reduces health care costs, and
prevents the development of multidrug-resistant
pathogens.5

10. Although the prevalence of life-threatening
anaphylactic reactions to penicillin has been estimated
to be between 0.02% and 0.04%

11. the most common reaction is a cutaneous eruption.

12. Since anaphylactic reactions are mediated by
immunoglobulin E (IgE), evaluation of patients with a
history of penicillin allergy by penicillin skin testing is
recommended to rule out IgE-mediated reactions.

13. EVALUATING ALLERGIC PATIENTS

14. Evaluation of patients with a history of penicillin
allergy can be improved with an understanding of the
classification of drug reactions, risk factors for allergy,
and the pathophysiology of penicillin allergy.

15. Adverse drug reactions include all unintended
pharmacologic effects of a drug and can be classified
as predictable (type A) or unpredictable (type B)

16. Predictable reactions are dose-dependent, are related
to the known pharmacologic actions of the
medication, and occur in otherwise healthy
individuals

17. Unpredictable reactions are further classified into
drug intolerance, drug idiosyncrasy, drug allergy, and
pseudoallergic reactions.8,9

18. Penicillin allergy can manifest as any hypersensitivity
reaction of the Gell and Coombs classification (TABLE
1).9

19. Gell and Coombs classification

20. Risk factors for IgE-mediated
reaction
 frequent or repetitive courses of penicillin10
 high-dose parenteral (rather than oral)
administration.
 Age and atopy are not risk factors for penicillin
allergy.7 However, atopy increases the risk of a more
severe anaphylactic reaction to penicillin, and
anaphylactic reactions are most commonly reported
between the ages of 20 and 49.6

21. Pathophysiology of penicillin
allergy
 All penicillins share a common core ring structure
(beta-lactam and thiazolidine rings) but differ in their
side chains (R group)

22.  Under physiologic conditions, the core ring structure
is metabolized into major (penicilloyl) and minor
(penicillin itself, penicilloate and penilloate) antigenic
determinants that may trigger an immediate IgE-
dependent response

23.  In the United States, commercial forms of antigenic
determinates for skin testing exist in the form of
penicillin G (minor determinant) and penicilloyl-
polylysine, better known as Prepen (major
determinant).

24.  Immediate-type reactions to similar antibiotics such
as aminopenicillins and cephalosporins may be caused
by IgE antibodies against the R-group side chain rather
than the core penicillin major and minor
determinants.11

26.  Immediate-type reactions to similar antibiotics such as
aminopenicillins and cephalosporins may be caused by
IgE antibodies against the R-group side chain rather
than the core penicillin major and minor
determinants.11

27. Do you remember the details of
the reaction?
 These include the route of administration, the time
between the dose of penicillin and the appearance of
symptoms, and how the reaction was managed.

28. Do you remember the details of
the reaction?
 If the patient has had severe non–IgE-mediated
reactions to penicillin (eg, Stevens-Johnson syndrome,
toxic epidermal necrolysis, acute interstitial nephritis,
hemolytic anemia, or serum sickness) in the past, skin
testing, graded-dose challenge, and desensitization are
contraindicated.

29. How many years ago did the
reaction occur?
 Most patients lose their sensitivity to penicillin over
time.7,13–15
 Nearly 50% of patients with IgE-mediated penicillin
allergy lose their sensitivity within 5 years of the
reaction,15 increasing to 80% or more by 10 years.13

30. How was the reaction managed?
What was the outcome?
 Use of and positive response to epinephrine and
histamine 1 receptor antagonists (antihistamines) with
resolution or significant improvement of symptoms
within a few hours may indicate an IgE-mediated
reaction.

31. What was the indication for
penicillin?
 Many cutaneous reactions are a result of an underlying
viral or bacterial infection.
 For example, up to 90% of patients with Epstein-Barr
virus infection develop a maculopapular rash when
given penicillin.16

32. Have you tolerated other forms of
penicillin since the reaction?
 Sometimes the patient has already tolerated other
beta-lactams such as aminopenicillins,
cephalosporins, and semisynthetic penicillins
(piperacillin-tazobactam).
 Patients who tolerate other beta-lactams without
adverse reactions are not allergic to beta-lactams.

33.  Diagnostic tests

34. Skin testing.
 The only validated test for diagnosing IgE-mediated
reactions caused by penicillin is the immediate
hypersensitivity skin test,9
 which should be performed by a board-certified
allergist.

35. Skin testing.
 The test consists of skin-prick and intradermal testing
 Results of skin-prick testing are read 15 minutes after application. A
positive response is a wheal at least 3 mm larger in diameter (with
equivalent erythema) than the negative control done simultaneously
 Intradermal testing is only done after a negative skin-
prick test

36. Skin testing.
 A history of severe non–IgE-mediated reaction to
penicillin is a contraindication to skin-prick testing for
penicillin allergy. The positive predictive value of
penicillin skin testing is 50%, and the negative
predictive value is 97%.3,7,9,13

37. Commercial in vitro testing
 (serum-specific IgE assays) for IgE-mediated
hypersensitivity to penicillin is inferior to skin testing
in terms of the negative predictive value and is not a
suitable substitute for penicillin skin testing.

38. MANAGING PENICILLIN ALLERGY
 If skin testing is positive, use another antibiotic,
or refer for desensitization
 If penicillin skin testing is positive use another
antibiotic that is equally efficacious.
 Patients who absolutely need a beta-lactam may
undergo drug desensitization, performed by a board-
certified allergist.

39. desensitization
 begin with a dose ranging from 1/10,000 to 1/1,000 of the final dose,
 progressively higher doses of the drug every 15 to 20 minutes subcutaneously or
intravenously, or every 20 to 30 minutes orally, until a full therapeutic dose is
tolerated
 Using modern protocols, the success rate for tolerance
induction is extremely high (75% to 100%
 Drug desensitization is contraindicated in patients
with non–IgE-mediated reactions.

40. graded-dose challenge
 If skin testing is negative, refer for graded-dose
challenge
 If skin testing is negative , graded-dose challenge is recommended. This
procedure must be done by a board-certified allergist. If the original
reaction was life-threatening, graded-dose challenge may entail giving
1/100 of the therapeutic dose. Then, if no reaction occurs during a brief
observation period (usually 30 minutes), a full dose is given.
 Graded-dose challenge is contraindicated if the
original reaction was a severe non–IgE-mediated
reaction.

41. UNDERSTANDING CROSS-
REACTIVITY OF PENICILLIN

42. UNDERSTANDING CROSS-
REACTIVITY OF PENICILLIN
 Penicillin is the only antibiotic for which skin testing is
reliable and validated.
 The rate of cross-reactivity between penicillin and
aminopenicillins (amoxicillin and ampicillin) is less
than 1.3%
 However, the cross-reactivity rate among
aminopenicillins and cephalosporins is between 10%
to 40%.

43. UNDERSTANDING CROSS-
REACTIVITY OF PENICILLIN
 The rate of cross-reactivity between penicillin and cephalosporins
was reported as 10% ,40 years ago.23,24
 But this was with early, first-generation cephalosporins that may
have been contaminated with penicillin.
 The cross-reactivity rate with cephalosporins today is
3%.25
 In general, first- and second-generation cephalosporins cause more allergic
reactions than third- and fourth-generation cephalosporins.26

44.  The rate of cross-reactivity between penicillin and
imipenem was less than 1%.
 Monobactam antibiotics do not cross-react with other
beta-lactams, except ceftazidime with aztreonam. This
is probably because of similarities in their chemical
structure.

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